1. Field of the Invention
The present invention generally relates to apparatus for measuring gravity variations on the earth surface and also in holes drilled into the earth and, more particularly, is concerned with a differential interferometric ballistic gravity measurement apparatus and method.
2. Description of the Prior Art
Relative gravity meters, based on the principle of balancing the weight of a fixed mass with forces from a normal or superconducting spring, are used in measuring gavity variations from point to point on the earth surface and also in holes drilled into the earth. Absolute gravity meters are known which use laser light interferometry to precisely measure the acceleration of a falling mass and thus the absolute value of gravity. Examples are disclosed in U.S. Pat. No. 5,351,122 to Niebauer et al and U.S. Pat. No. 5,461,914 to Zumberge et al. The gravity meters of these patents have one arm of the interferometer in free-fall and the other arm stationary and isolated from ground motion. The gravity meter of the Zumberge et al patent employs a fiber-optic element to direct the laser light and uses the end of the fiber to act as a beam-splitter.
Also, methods for measuring gravity gradient over a fixed vertical interval are described in U.S. Pat. No. 3,693,451 to Dunlap et al, U.S. Pat. Nos. 3,688,584 and 3,727,462 to Stone et al, and U.S. Pat. No. 3,704,626 to Stone. Measuring gravity gradient, as done with the devices disclosed in the Stone patents, implies the fixed positioning of two masses for free-falling in spaced relation along the same vertical axis and measuring gravity at two vertically displaced points.
The devices mentioned above for measuring relative and absolute gravity and gravity gradient have the following drawbacks when used in many practical applications. Many times, they are too large in size to be useful, too time-consuming to setup and calibrate, and too sensitive to environmental conditions that adversely affect their performance and the reliability of their measurements. Relative gravity meters of the spring-type which have been used extensively in boreholes are but one example. They have size and calibration problems; and they are sensitive to temperature changes. Absolute gravity meters also typically suffer from similar problems which limit their range of applications. Both relative and absolute gravity meters are also time-dependent instruments, requiring measurements of gravity to be taken sequentially. The gradient gravity apparatus described above operates only with respect to vertical gradient.
Consequently, a need exists for innovation in the design and development of a gravity measurement apparatus which will retain beneficial features of some of these prior art meters and apparatus but not their drawbacks nor substitute new drawbacks in their place.